1. Field of the Invention
This invention generally relates to a splined ball screw assembly including a spiral guide channel, which is defined between a pair of inner and outer members and provided with a plurality of balls, and also one or more axial guide channels provided with a plurality of balls so as to allow to provide a relative axial movement between the inner and outer members. More particularly, the present invention relates to a splined ball screw assembly having a nested structure in which the spiral and axial guide channels are provided on the opposite peripheral surfaces of a hollow shaft.
2. Description of the Prior Art
A typical prior art splined ball screw assembly is described in the U.S. Pat. No. 3,046,808 and its representative structure is illustrated in FIG. 6. As shown, the splined ball screw assembly generally includes a shaft 26 which is formed with a ball screw section 70 and a ball spline section 71 as located side-by-side in the axial direction of the shaft 26. A spiral guide groove and a plurality of axial guide grooves are formed on the same portion of the shaft 26. The assembly also includes a nut 20 which is fitted onto the shaft 26 with a predetermined gap therebetween and a plurality of balls partly engaged with the spiral guide groove of the shaft 26 and partly engaged with the spiral guide groove of the nut 20. The assembly also includes an outer sleeve 72 which is located in a side-by-side relation with the nut 20 in the axial direction of the shaft 26 and which includes a plurality of endless guide passages filled with balls which are partly engaged with the respective axial guide grooves of the shaft when located at the load section of the endless guide passage.
With the above-described structure, when the nut 20 is driven to rotate in either direction through an associated drive gear train, the shaft 26 is caused to move in either direction along its axial direction without rotation through a spline engagement.
However, in the above-described prior art structure, since the ball screw section 70 and the ball spline section 71 are located in a side-by-side arrangement in the axial direction, the overall axial length of the completed assembly tends to be large. Thus, there is a limit in making the overall assembly compact in size. In addition, since both of the spiral and axial guide grooves are formed on the same portion of the shaft, processing tends to be complicated and care and high precision are required in processing, so that a relatively long period of time is required and an advanced processing technology is required, which tends to push up the cost.